1. Field of the Invention
This invention pertains to the field of secondary and/or tertiary recovery of petroleum from a subterranean hydrocarbon reservoir utilizing as a recovery medium emulsified mixtures of hydrocarbons, aqueous fluids and surfactants. These recovery mediums are generally known as micellar dispersions.
2. Description of the Prior Art
The prior art teaches that solubilized oil-water systems can be employed as a type of solvent slug in miscible flooding operations to recover oil from subterranean hydrocarbon formations. In a typical operation of this kind the leading edge of the slug is miscible with the oil bank ahead of it, and the water or aqueous components used in the driving slug is miscible, or at least partially soluble, in the trailing edge of the solvent slug. this in effect forms a single phase flooding operation.
A problem that has plagued the use of emulsified and/or micellar dispersions in petroleum recovery operations has been their limited use in conjunction with, or in formations containing hard water. Hard water results from the presence of large amounts of polyvalent cations such as calcium or magnesium. This disadvantage, or course, not only limits the kind of aqueous fluid which may be used to form the micellar dispersion but also limits the types of reservoirs in which the micellar dispersions may be used. For example, U.S. Pat. No. 3,506,070 to Jones describes a micellar dispersion but specifically states that soft water is preferred and limits the amount of dissolved salts to small amounts. In an example in the Jones patent a micellar dispersion is formed with water containing only 412 parts per million of total dissolved solids. This amount of dissolved solids is of course quite small and difficult to obtain in oil fields. Thus, the type of dispersion described in Jones would not be useful in a formation containing large amounts of dissolved polyvalent cations such as a calcium carbonate reservoir. U.S. Pat. No. 3,698, 479 to Askew also describes a micellar dispersion. However, Askew specifically states that the presence of excessive amounts of divalent metal ions in the system in which the micellar dispersion is formed is undesirable. U.s. Pat. No. 3,688,844 to Roszelle attempts to solve the problem of limited use of micellar dispersions in hard waters by adding a metal phosphate to the micellar dispersion. Applicants have discovered certain surfactant systems whereby micellar dispersions may be formed which have a very high degree of tolerance for very hard water without the addition of special additives.